This invention relates to improvements in piston-driven valves. It is known that such valve units include a valve body equipped with inlet and outlet openings the communications between which may be interrupted by a clack-valve which is integral with a rod connected to a piston which is subjected to a valve control pressure. Such valves chiefly are of two types:
(a) "normally closed" valves, where the driving piston, when at rest and subjected the action of a spring, applies the clack-valve on a valve seat, so that the valve opens only when a control pressure acts against the spring;
(b) "normally open" valves, where the driving piston, when at rest and subjected to the action of a spring, keeps the clack-valve away from the valve seat, so that the valve closes only when the control pressure acts against the spring.
There are also piston-driven valves which are either normally open or normally closed, working under either normal or low pressure. Thus, the operator must have at his disposal a relatively high number of valves for the various applications contemplated.